1. Field of the Invention
The present invention relates to cell culture methods and compositions that are essentially serum-free and comprise a basal salt nutrient solution and an ErbB3 ligand.
2. Background of the Invention
Human pluripotent cells offer unique opportunities for investigating early stages of human development as well as for therapeutic intervention in several disease states, such as diabetes mellitus and Parkinson's disease. For example, the use of insulin-producing β-cells derived from human embryonic stem cells (hESCs) would offer a vast improvement over current cell therapy procedures that utilize cells from donor pancreases. Currently cell therapy treatments for diabetes mellitus, which utilize cells from donor pancreases, are limited by the scarcity of high quality islet cells needed for transplant. Cell therapy for a single Type I diabetic patient requires a transplant of approximately 8×108 pancreatic islet cells (Shapiro et al., 2000, N Engl J Med 343:230-238; Shapiro et al, 2001a, Best Pract Res Clin Endocrinol Metab 15:241 264; Shapiro et al, 2001, British Medical Journal 322:861). As such, at least two healthy donor organs are required to obtain sufficient islet cells for a successful transplant.
Embryonic stem (ES) cells thus represent a powerful model system for the investigation of mechanisms underlying pluripotent cell biology and differentiation within the early embryo, as well as providing opportunities for genetic manipulation of mammals and resultant commercial, medical and agricultural applications. Furthermore, appropriate proliferation and differentiation of ES cells can potentially be used to generate an unlimited source of cells suited to transplantation for treatment of diseases that result from cell damage or dysfunction. Other pluripotent cells and cell lines including early primitive ectoderm-like (EPL) cells as described in International Patent Application WO 99/53021, in vivo or in vitro derived ICM/epiblast, in vivo or in vitro derived primitive ectoderm, primordial germ cells (EG cells), teratocarcinoma cells (EC cells), and pluripotent cells derived by dedifferentiation or by nuclear transfer will share some or all of these properties and applications. International Patent Application WO 97/32033 and U.S. Pat. No. 5,453,357 describe pluripotent cells including cells from species other than rodents. Human ES cells have been described in International Patent Application WO 00/27995, and in U.S. Pat. No. 6,200,806, and human EG cells have been described in International Patent Application WO 98/43679.
The biochemical mechanisms regulating ES cell pluripotency and differentiation are very poorly understood. However, the limited empirical data available (and much anecdotal evidence) suggests that the continued maintenance of pluripotent ES cells under in vitro culture conditions is dependent upon the presence of cytokines and growth factors present in the extracellular milieu.
While human ESCs offer a source of starting material from which to develop substantial quantities of high quality differentiated cells for human cell therapies, these cells must be obtained and/or cultured in conditions that are compatible with the expected regulatory guidelines governing clinical safety and efficacy. Such guidelines likely will require the use of a chemically defined media. The development of such chemically defined/GMP standard conditions is necessary to facilitate the use of hESCs and cells derived from hESCs for therapeutic purposes in humans.
In addition, the eventual application of hESC based cell replacement therapies will require the development of methods that enable large scale culture and differentiation conditions that are compliant with regulatory guidelines. While several groups have reported simplified growth conditions for hESCs, there are substantial limitations with these studies. To date, however, the successful isolation, long-term clonal maintenance, genetic manipulation and germ line transmission of pluripotent cells has generally been difficult.
Most of the cell culture conditions for stem cells still contain serum replacer (KSR) in the media (Xu et al., 2005 Stem Cells, 23:315-323; Xu et al., 2005 Nature Methods, 2:185-189; Beattie et al., 2005 Stem Cells, 23:489-495; Amit et al., 2004 Biol. Reprod., 70:837-845; James et al., 2005 Development, 132:1279-1282). KSR contains a crude fraction of bovine serum albumin (BSA) rather than a highly purified source. Others have only performed short-term studies, and therefore it is not clear if their conditions would enable the maintenance of pluripotency over extended periods (Sato et al., (2004) Nature Med., 10:55-63; U.S. Patent Publication Nos. 2006/0030042 and 2005/0233446). Others have shown long-term maintenance of pluripotency in a chemically defined media with FGF2, activin A, and insulin, but the cells were grown on plates that were coated with human serum, which was “washed off” before plating of cells (Vallier et al., 2005 J Cell Sci., 118(Pt 19):4495-509). While FGF2 has been a component of all these media, it is not clear if it is an absolute necessity, particularly as in some formulations it is necessary to use it at a high concentration (up to 100 ng/ml, Xu et al., 2005 Nature Methods, 2:185-189).
Furthermore, all of these groups have either included insulin in their media at μg/ml levels, or have insulin present due to the use of KSR. Insulin is typically considered to function in glucose metabolism and “cell survival” signaling via binding to the insulin receptor. At levels above physiological concentrations, however, insulin can also bind to the IGF1 receptor with a lower efficiency and confer classical growth factor activity through the PI3 Kinase/AKT pathway. The presence/requirement for such high levels of insulin (μg/ml levels) in KSR or these other media conditions suggests that the major activity is elicited via binding to the IGF1 receptor, which is expressed by hESCs (Sperger et al., 2003 PNAS, 100(23):13350-13355). Others have noted the expression of a full complement of IGF1R and intracellular signaling pathway members in hESCs, which is likely to signify the functional activity of this pathway (Miura et al., 2004 Aging Cell, 3:333-343). Insulin or IGF1 may elicit a major signal required for the self-renewal of hESCs, as is suggested by the fact that all conditions developed thus far for the culture of hESC contain either insulin, insulin provided by KSR, or IGF1 provided by serum. In support of this concept, it has been shown that if PI3 Kinase is inhibited in hESC cultures, the cells differentiate (D'Amour et al., 2005 Nat. Biotechnol., 23(12):1534-41; McLean et al., 2007 Stem Cells 25:29-38).
A recent publication outlines a humanized, defined media for hESCs (Ludwig et al., Nature Biotechnology, published online Jan. 1, 2006, doi:10.1038/nbt1177). This recent formulation, however, includes several factors that are suggested to influence the proliferation of hESCs, including FGF2, TGFβ, LiCl, γ-aminobutyric acid and pipecolic acid. It is noted that this recently defined cell culture medium also contains insulin.
The EGF growth factor family has at least 14 members, including, but not limited to, EGF, TGFβ, heparin binding-EGF (hb-EGF), neuregulin-β (also named heregulin-β (HRG-β), glial growth factor and others), HRG-α, amphiregulin, betacellulin, and epiregulin. All these growth factors contain an EGF domain and are typically first expressed as transmembrane proteins that are processed by metalloproteinase (specifically, ADAM) proteins to generate soluble ectodomain growth factors. EGF family members interact with either homo- or hetero-dimers of the ErbB1, 2, 3 and 4 cell surface receptors with different affinities (Jones et al., FEBS Lett, 1999, 447:227-231). EGF, TGFα and hbEGF bind ErbB1/1 (EGFR) homodimers and ErbB1/2 heterodimers at high affinity (1-100 nM range), whereas HRG-β binds ErbB3 and ErbB4 at very high affinity (<1 nM range). Activated ErbB receptors signal through the PI3 Kinase/AKT pathway and also the MAPK pathway. ErbB2 and ErbB3 are amongst the most highly expressed growth factor receptors in hESCs (Sperger et al., 2003 PNAS, 100(23):13350-13355) and HRG-β has been shown previously to support the expansion of mouse primordial germ cells (Toyoda-Ohno et al., 1999 Dev. Biol., 215(2):399-406). Furthermore, overexpression and subsequent inappropriate activation of ErbB2 is associated with tumorigenesis (Neve et al., 2001 Ann. Oncol., 12 Suppl 1:S9-13; Zhou & Hung, 2003 Semin. Oncol., 30(5 Suppl 16):38-48; Yarden, 2001 Oncology, 61 Suppl 2:1-13). Human ErbB2 (Chromosome 17q), and ErbB3 (Chromosome 12q) are present on chromosomes that have been observed to accumulate as trisomies in some hESCs (Draper et al., 2004 Nat. Biotechnol., 22(1):53-4; Cowan et al., 2004 N Engl. J. Med., 350(13):1353-6; Brimble et al., 2004 Stem Cells Dev., 13(6):585-97; Maitra et al., 2005 Nat. Genet. 37(10):1099-103; Mitalipova et al., 2005 Nat. Biotechnol. 23(1): 19-20; Draper et al., 2004 Stem Cells Dev., 13(4):325-36; Ludwig et al., Nature Biotechnology, published online Jan. 1, 2006, doi:10.1038/nbt1177).
ErbB2 and ErbB3 (Brown et al., 2004 Biol. Reprod., 71:2003-2011; Salas-Vidal & Lomeli, 2004, Dev Biol., 265:75-89) are expressed in the mouse blastocyst, although not specifically restricted to the inner cell mass (ICM), and ErbB1, EGF and TGFβ are expressed in the human blastocyst (Chia et al., 1995 Development, 1221(2):299-307). HB-EGF has proliferative effects in human IVF blastocyst culture (Martin et al., 1998 Hum. Reprod., 13(6): 1645-52; Sargent et al., 1998 Hum. Reprod. 13 Suppl 4:239-48), and modest additional effects on mouse ES cells grown in 15% serum (Heo et al., 2005, Am. J. Phy. Cell Physiol., in press). Pre- and early post-implantation development does not appear to be affected in ErbB2−/−, ErbB3−/−, Neuregulin1−/− (Britsch et al., 1998 Genes Dev., 12:1825-36), ADAM17−/− (Peschon, et al., 1998 Science, 282: 1281-1284) and ADAM19−/− (Horiuchi, 2005 Dev. Biol., 283(2):459-71) null embryos. Therefore, the importance of signaling through the ErbB receptor family in hESCs is, up to now, unclear.
Neuregulin-1 (NRG1) is a large gene that exhibits multiple splicing and protein processing variants. This generates a large number of protein isoforms, which are referred to herein collectively as neuregulin. Neuregulin is predominantly expressed as a cell surface transmembrane protein. The extracellular region contains an immunoglobulin-like domain, a carbohydrate modified region and the EGF domain. NRG1 expression isoforms have been reviewed previously (Falls, 2003 Exp. Cell Res., 284: 14-30). The cell membrane metalloproteases ADAM17 and ADAM19 have been shown to process the transmembrane form(s) of neuregulin-1 to soluble neuregulin/heregulin. HRG-α and -β are the cleaved ectodomains of neuregulin, containing the EGF and other domains. As the EGF domain is responsible for binding and activation of the ErbB receptors, a recombinant molecule containing only this domain can exhibit essentially all of the soluble growth factor effects of this protein (Jones et al., 1999 FEBS Lett., 447:227-231). Also, there are processed transmembrane isoforms of neuregulin that are thought to trigger juxtacrine signaling in adjacent cells via interaction of the EGF domain with ErbB receptors.
An important development in the progression of hESC research toward maintaining pluripotency in culture will be the elucidation of media and cell culture conditions that are compatible with the expected regulatory guidelines governing clinical safety and efficacy. While the best outcome would be the availability of chemically defined media for hESCs, components that are not chemically defined would be acceptable if they were produced to GMP standard. There is a need, therefore, to identify methods and compositions for the culture and stabilization of a population of pluripotent stem cells that are able to be used for therapeutic purposes, wherein the culture compositions are defined and/or produced to GMP standard.